Police are offering a $50,000 reward in their hunt for the murderer of Auckland teenager Jane Furlong, who disappeared 20 years ago.

Detective Inspector Mark Benefield told a press conference this afternoon that there were a number of people who were refusing to talk to police and police believed the reward would give them an incentive to speak up.

The reward will be for information that leads to the identification and conviction of those involved in Furlong's death.

Furlong was last seen while working as a prostitute on Karangahape Rd, Auckland on May 28, 1993. She left behind a 6-month-old baby, Aidan Norsworthy, who is now 20.

The 17-year-old's remains were found on Sunset Beach in Port Waikato on May 19, last year, by a woman walking her dog and were identified by police through DNA testing.

A homicide investigation was launched at the time of the discovery and police now believe more than one person was involved in the part-time sex worker's death.

Today, Benefield said one of the people who had declined to talk to police was Jane's boyfriend at the time of her disappearance, Danny Norsworthy, and others included those involved in the court cases Jane was going to be witness to.

When Furlong disappeared she was due to testify as a witness for the prosecution in two separate cases - one involved gang members accused of an assault, the other a businessman charged with brutal attacks on sex workers.

"We have interviewed and spoken with literally hundreds of people, some of whom have been less helpful than others," Benefield said.

"We know there are people who know what happened to Jane after she was reportedly last seen and those people owe it to themselves, Jane's memory, her family and friends to tell us what they know."

Benefield has previously said it was possible Furlong was alive for some days after she disappeared.

He earlier appealed for a man recorded in Furlong's diary as "Tommy", whom it is believed Furlong may have planned to spend a night with near the time of her disappearance, to come forward, along with any other clients who could recall being with her on, or after May 26.

A large billboard advising of the $50,000 reward offer has been rented at the western end of Karangahape Rd and will be installed this afternoon.

It will remain there for six weeks and anyone with information is urged to contact the investigation team on 0800 675 263 or e-mail furlong@police.govt.nz.